This invention relates to apparatus for starting and then operating high-pressure sodium discharge lamps and, more particularly, to such apparatus which provides high-voltage pulses to start such lamps even at relatively low-line voltage.
A number of discharge lamp lighting circuits have been developed in recent years and one such apparatus is described in copending application Ser. No. 540,185, filed on Jan. 10, 1975 by Joseph C. Engel and Gary F. Saletta, and owned by the present assignee. The apparatus described in this copending application provides for high-voltage pulses for starting a sodium discharge lamp by using the breakdown characteristics of a Zener diode to provide accurately timed starting pulses. The use of the Zener diode eliminates timing problems encountered when the breakdown characteristics of a glow lamp are used in somewhat similar fashion, as described in U.S. Pat. No. 3,917,976 issued Nov. 4, 1975 to Nuckolls and U.S. Pat. No. 3,963,958 issued June 5, 1976 to Nuckolls. The circuit described in the aforementioned copending application and the prior art described in the aforesaid Nuckolls' patent issued June 15, 1976, function satisfactorily in many applications, although difficulties are encountered when ballast reactors of low open circuit voltage are used. To compensate for this problem, a larger than desirable storage capacitor and a smaller charging resistor could be used which in turn creates a problem of diverting some of the lamp starting high-voltage pulse. The aforementioned Nuckolls' patent issued Nov. 4, 1975 used an R.F. choke in series with the charging resistor to offer a higher impedance to the generated high voltage pulse, thereby transferring more starting power to the lamp. This in turn increased the cost of the starting circuit.